Many computing scenarios involve the issuing of an operation, such as a query, to a server to be performed on behalf of a client. As one example, a service may accept a relational query to be applied against a data store, and may respond with a message indicating the success or failure of the operation and/or a result data set. Such services may be invoked synchronously (where the client blocks while waiting for the server to complete the service) or asynchronously (where the client continues to perform some other actions while the operation is pending, and where the client is later notified or detects that the operation is complete.) Moreover, the server may concurrently handle many transactions on behalf of one or more clients, and may have access to a finite (though potentially very large) set of resources (e.g., processing bandwidth, memory, and network bandwidth) for performing the queries on behalf of the clients. The taxing of the resources may result in diminished performance and latency in completing such operations; if the queue of operations to be performed grows too large, this may result in failed operations and/or incorrect results.